Integrated thermochemical reactors/heat exchangers for solar energy storage based on porous ceramic structures

Kurzfassung

Thermochemical Storage (TCS) of solar energy exploits the heat effects of reversible chemical reactions. Solar heat produced during on-sun operation of Concentrated Solar Power (CSP) plants is used to power an endothermic chemical reaction; if this reaction is completely reversible the thermal energy can be entirely recovered by the reverse reaction during off-sun operation. Among such possible reversible gas-solid chemical reactions, the utilization of a pair of reduction-oxidation (redox) reactions of solid oxides of multivalent metals can be directly coupled to CSP plants employing air as the heat Transfer fluid. Challenges in the development of real-size thermochemical storage systems based on such oxidebased redox materials lie in the one hand in tailoring their heat storage/release capability and in the other hand in maximizing heat transfer characteristics between the solid particles and the air stream, in other words, in designing and operating TCS reactors simultaneously and efficiently as heat exchangers. Thermo-Gravimetric Analysis experiments have demonstrated that the redox pair Co3O4/CoO is one of the most attractive for this application since it can operate in a completely reversible, cyclic redox mode within the temperature range 800-1000°C. Subsequently, stemming from the establishment of structured reactors based on porous ceramics like honeycombs and foams in a plethora of chemical engineering applications, the idea of employing similar structures coated with or manufactured entirely from Co3O4 has been set forth. Lab-scale Co3O4-coated, ceramic foams and honeycombs have also demonstrated such redox operation, exploiting the entire amount of coated Co3O4 even at levels exceeding 100 wt% loading. To enhance volumetric energy yield, monolithic porous structures like pellets and foams made entirely of Co3O4 were manufactured and similarly tested. During cycling, such structures also exploited the entire amount of Co3O4 for redox reactions; however they were subjected not only to thermally- but also to “chemically”-induced stresses due to the expansion-contraction of the cobalt oxide lattice during oxygen release/uptake. Under such conditions, “open” porous structures like the ones of ceramic foams seem to have several advantages. Current efforts are targeted to maximizing their redox performance maintaining simultaneously their structural integrity.